1. Field of the Invention
The present invention relates to a liquid crystal display device and an electronic device.
2. Description of the Related Art
In recent years, techniques for manufacturing semiconductor devices using a semiconductor thin film which is formed over a substrate having an insulating surface have been researched. Semiconductor devices using the above-mentioned semiconductor thin film are widely used, and in particular, are often used as switching elements for image display devices.
As a switching element of the image display device, an amorphous semiconductor or a polycrystalline semiconductor is used. In the case of using the amorphous semiconductor, since the number of the steps can be reduced by eliminating the steps necessary for crystallization or the like, there is an advantage that the switching element can be manufactured with low cost. In the case of using the polycrystalline semiconductor, there is an advantage that high functional switching element can be manufactured.
Examples of the image display device using the switching element include a liquid crystal display device or an electroluminescence display device. Liquid crystal display device is a device which performs display by changing an orientation of liquid crystal molecules due to applying the potential difference between a pixel electrode and a counter electrode. Note that a storage capacitor is generally provided in a pixel in order to hold the potential difference which is necessary for display for a certain period. An electroluminescence display device performs display in such a manner that electric charges are supplied to a light-emitting material provided between electrodes and radiative recombination of carriers is derived.
In liquid crystal display device, the aperture ratio of a pixel is one of the important parameters for determining display quality. The luminance increases by improving the aperture ratio, whereby display of high contrast is realized. In addition, the output of a backlight can be reduced by improving the aperture ratio. A method of improving aperture ratio includes, for example, a method in which a storage capacitor is formed using a light shielding film (for example, Reference 1: Japanese Published Patent Application No. H10-170961).
In the meantime, as a semiconductor material, there is a microcrystalline semiconductor as well as an amorphous semiconductor and a polycrystalline semiconductor. For example, the microcrystalline silicon is known materials for a long time as well as the amorphous silicon. History of a field effect transistor using the microcrystalline silicon can go back to 1980's (For example, Reference 2: U.S. Pat. No. 5,591,987). However, a transistor using the microcrystalline silicon has lagged behind a transistor using the amorphous silicon and a transistor using the polycrystalline silicon in practical application till now, and reports are found only in academic conferences and so on (for example, Reference 3: Toshiaki Arai et al., SID 07 DIGEST, 2007, pp. 1370-1373).